Negative-electrode active material and preparation method thereof, secondary battery, and battery module, battery pack, and apparatus containing such secondary battery

ABSTRACT

This application discloses a negative-electrode active material and a preparation method thereof, a secondary battery, and a battery module, a battery pack, and an apparatus that include such secondary battery. The negative-electrode active material includes a core and a coating layer covering at least part of a surface of the core, where the core includes artificial graphite, the coating layer includes amorphous carbon, a volume-based particle size distribution of the negative-electrode active material satisfies Dv99≤24 μm, a volume-based median particle size Dv50 of the negative-electrode active material satisfies 8 μm≤Dv50≤15 μm, Dv99 is a particle size corresponding to a cumulative volume distribution percentage of the negative-electrode active material reaching 99%, and Dv50 is a particle size corresponding to a cumulative volume distribution percentage of the negative-electrode active material reaching 50%.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 17/822,260, filed on Aug. 25, 2022, which is a continuation ofInternational Patent Application No. PCT/CN2020/121268, filed on Oct.15, 2020. The aforementioned patent applications are incorporated hereinby reference in their entirety.

TECHNICAL FIELD

This application relates to the field of secondary battery technologies,and in particular, to a negative-electrode active material and apreparation method thereof, a secondary battery, and a battery module, abattery pack, and an apparatus that include such secondary battery.

BACKGROUND

Secondary batteries are charged and discharged through repeatedintercalation and deintercalation of active ions between a positiveelectrode and a negative electrode, featuring outstanding features suchas high energy density, long cycle life, no pollution, and no memoryeffect. Therefore, as clean energy, the secondary batteries have beengradually popularized from electronic products to large-scaleapparatuses such as electric vehicles to adapt to sustainabledevelopment strategies of environment and energy.

However, compared with conventional oil-fueled vehicles that can berefueled quickly in a timely manner, the electric vehicles are generallycharged at a smaller rate, often requiring a longer charging time. Thiscauses range anxiety for consumers and limits rapid popularization ofthe electric vehicles. Therefore, in order to improve marketcompetitiveness of the electric vehicles, it is necessary to providesecondary batteries with good fast-charging performance.

SUMMARY

This application is intended to provide a negative-electrode activematerial and a preparation method thereof, a secondary battery, and abattery module, a battery pack, and an apparatus that include suchsecondary battery, so as to improve charging performance and cyclingperformance of secondary batteries.

In order to achieve the foregoing invention objective, a first aspect ofthis application provides a negative-electrode active material, whichincludes a core and a coating layer covering at least part of a surfaceof the core, where the core includes artificial graphite, the coatinglayer includes amorphous carbon, a volume-based particle sizedistribution of the negative-electrode active material satisfiesD_(v)99≤24 μm, and a volume-based median particle size D_(v)50 satisfies8 μm≤D_(v)50≤15 μm. D_(v)99 is a particle size corresponding to acumulative volume distribution percentage of the negative-electrodeactive material reaching 99%; and D_(v)50 is a particle sizecorresponding to a cumulative volume distribution percentage of thenegative-electrode active material reaching 50%.

It is surprisingly found that when the negative-electrode activematerial of this application is used for a negative-electrode plate, thenegative-electrode plate has higher active ion solid-phase diffusionperformance. Even in a high lithium intercalation state during a latercharging phase, active ions can have a relatively high diffusion rate inthe negative-electrode plate to effectively reduce ohmic andconcentration polarization. In this way, an overall charging speed andcharging depth of the negative-electrode plate are greatly increased,significantly improving a fast-charging capability of the battery.Further, the cycling performance of the battery is also significantlyimproved.

In any implementation of this application, the negative-electrode activematerial satisfies 17 μm≤D_(v)99≤24 μm, optionally, 18 μm≤D_(v)99≤21 μm.The negative-electrode active material with D_(v)99 being within theforegoing range can further improve the fast-charging capability andcycling performance of the battery.

In any implementation of this application, the negative-electrode activematerial satisfies 9 μm≤D_(v)50≤13 μm, optionally, 11 μm≤D_(v)50≤13 μm.The negative-electrode active material with D_(v)50 being within anappropriate range can further improve the fast-charging capability andcycling performance of the battery.

In any implementation of this application, a particle size uniformity ofthe negative-electrode active material is 0.25-0.45, optionally,0.32-0.38. The negative-electrode active material with the particle sizeuniformity being within the foregoing range can further improve thefast-charging capability of the battery; and further enables thenegative-electrode plate to obtain a higher compacted density, therebyimproving energy density of the battery.

In any implementation of this application, a particle size specificsurface area of the negative-electrode active material is 0.4 μm²/g-0.75μm²/g, optionally, 0.5 μm²/g-0.65 μm²/g. The negative-electrode activematerial with the specific surface area of particles being within theappropriate range can further improve the fast-charging performance andcycling performance of the battery and also increase an energy densityof the battery.

In any implementation of this application, the negative-electrode activematerial includes secondary particles, and a quantity proportion of thesecondary particles in the negative-electrode active material is ≥50%.Optionally, the quantity proportion of the secondary particles in thenegative-electrode active material is 70%-95%. The negative-electrodeactive material containing an appropriate quantity of secondaryparticles can further improve the fast-charging capability, cyclingperformance, and storage performance of the battery.

In any implementation of this application, the negative-electrode activematerial satisfies 0.6≤(D_(v)90−D_(v)10)/D_(v)50≤1.8, optionally,0.8≤(D_(v)90−D_(v)10)/D_(v)50≤1.4. D_(v)90 is a particle sizecorresponding to a cumulative volume distribution percentage of thenegative-electrode active material reaching 90%; and D_(v)10 is aparticle size corresponding to a cumulative volume distributionpercentage of the negative-electrode active material reaching 10%. Thenegative-electrode active material with (D_(v)90−D_(v)10)/D_(v)50 beingwithin an appropriate range helps further improve the fast-chargingcapability of the battery.

In any implementation of this application, the volume-based particlesize distribution D_(v)90 of the negative-electrode active material is13 μm-18 μm, optionally, 14 μm-17 μm. The negative-electrode activematerial with D_(v)90 being within the foregoing range can furtherimprove the fast-charging capability of the battery.

In any implementation of this application, the volume-based particlesize distribution D_(v)10 of the negative-electrode active material is 5μm-10 μm, optionally, 6 μm-8 μm. The negative-electrode active materialwith D_(v)10 being within the foregoing range helps improve the cyclingperformance and storage performance of the battery.

In any implementation of this application, a graphitization degree ofthe negative-electrode active material is 91.0%-96.0%, optionally,94.0%-95.0%. The negative-electrode active material with thegraphitization degree being within the foregoing range can furtherimprove the fast-charging capability of the battery.

In any implementation of this application, a gram capacity of thenegative-electrode active material is 345 mAh/g-360 mAh/g, optionally,350 mAh/g-358 mAh/g. The negative-electrode active material with thegram capacity being within an appropriate range can increase the energydensity of the battery and further improve the fast-charging capabilityand cycling performance of the battery.

In any implementation of this application, a tap density of thenegative-electrode active material is 0.9 g/cm³-1.3 g/cm³, optionally,1.0 g/cm³-1.1 cm³. The negative-electrode active material with the tapdensity being within the given range can improve the fast-chargingcapability of the battery and also increase the energy density of thebattery.

In any implementation of this application, a powder compacted density ofthe negative-electrode active material under a pressure of 2 kN is 1.55g/cm³-1.67 g/cm³, optionally, 1.60 g/cm³-1.65 g/cm³. Thenegative-electrode active material with the powder compacted densityunder the pressure of 2 kN being within the given range can implementclose contact between the particles of the negative-electrode film layerto form good electrolyte infiltration pore channels, thereby improvingthe fast-charging capability and cycling performance of the battery.

A second aspect of this application provides a preparation method fornegative-electrode active material, including the following steps:

-   -   (A) providing a core, where the core includes artificial        graphite; and    -   (B) coating the core to form a coating layer on at least part of        a surface of the core, so as to obtain a negative-electrode        active material, where the coating layer includes amorphous        carbon, and the negative-electrode active material satisfies        D_(v)99≤24 μm and 8 μm≤D_(v)50≤15 μm.

In any embodiment of this application, the preparation of artificialgraphite described in step (A) includes:

-   -   (a) providing a coke raw material;    -   (b) performing shaping processing on the coke raw material to        obtain a precursor;    -   (c) granulating the precursor to obtain a granulated product;        and    -   (d) graphitizing the granulated product to obtain artificial        graphite, where a volume-based median particle size D_(v)50 of        the artificial graphite is 6 μm-14 μm, and a volume-based        particle size distribution D_(v)99 is 17 μm-26 μm.

In any implementation of this application, a volume-based medianparticle size D_(v)50 of the granulated product is 9 μm-15 μm, and avolume-based particle size distribution D_(v)99 is 17 μm-24 μm.

In any implementation of this application, a volume-based medianparticle size D_(v)50 of the precursor is 8 μm-13 μm, and a volume-basedparticle size distribution D_(v)99 is 16 μm-22 μm.

In any implementation of this application, a volume-based medianparticle size D_(v)50 of the coke raw material is 7 μm-12 μm, and avolume-based particle size distribution D_(v)99 is 15 μm-21 μm.

In any implementation of this application, a particle size uniformity ofthe precursor is denoted by U₁, and satisfies 0.2≤U₁≤0.55, optionally,0.3≤U₁≤0.45.

In any implementation of this application, a particle size uniformity ofthe artificial graphite is denoted by U₂, and satisfies 0.22≤U₂≤0.48,optionally, 0.3≤U₂≤0.4.

In any implementation of this application, a volatile content of thecoke raw material is denoted by C₁, and the particle size uniformity ofthe precursor is denoted by U₁; and a binder is added during granulationin the step (c), an amount of the binder is denoted by C₂, and thepreparation method satisfies: 21%≤(C₁+C₂)/U₁×100% 50%, optionally, 31%K(C₁+C₂)/U₁×100%≤35%.

In any implementation of this application, the volatile content C₁ ofthe coke raw material satisfies 1%≤C₁≤12%, optionally, 5%≤C₁≤9%.

In any implementation of this application, the coke raw materialincludes one or more of petroleum-based non-needle coke andpetroleum-based needle coke; and optionally, the coke raw materialincludes petroleum green coke.

In any implementation of this application, the step (B) includes: (e)coating the core with an organic carbon source and performing heattreatment to form an amorphous carbon coating layer on at least part ofthe surface of the core, so as to obtain the negative-electrode activematerial.

In any implementation of this application, an amount of the organiccarbon source added in the step (e) is denoted by C₃; and thepreparation method satisfies: 20%≤(C₁+C₂+C₃)/U₂×100%≤56%, and1.2%≤C₃×carbon residue rate≤2.5%.

The negative-electrode active material obtained by using the preparationmethod of this application includes the core and the coating layercovering the surface of the core, the core includes artificial graphite,the coating layer includes amorphous carbon, and the negative-electrodeactive material satisfies D_(v)99≤24 μm and 8 μm≤D_(v)50≤15 μm, therebysignificantly improving the fast-charging capability of the batteryusing the negative-electrode active material. Further, the cyclingperformance of the battery is also significantly improved.

A third aspect of this application provides a secondary battery, whichincludes a negative-electrode plate, and the negative-electrode plateincludes a negative-electrode current collector and a negative-electrodefilm layer that is provided on at least one surface of thenegative-electrode current collector and that includes anegative-electrode active material, where the negative-electrode activematerial includes the negative-electrode active material described inthis application.

The secondary battery of this application uses the negative-electrodeactive material described in this application, and therefore cansimultaneously implement relatively high energy density, fast-chargingcapability, and cycling performance.

A fourth aspect of this application provides a battery module, includingthe secondary battery in this application.

A fifth aspect of this application provides a battery pack, includingthe secondary battery or the battery module in this application.

A sixth aspect of this application provides an apparatus, including atleast one of the secondary battery, the battery module, or the batterypack in this application.

The battery module, the battery pack, and the apparatus in thisapplication include the secondary battery provided in this application,and therefore have at least advantages the same as those of thesecondary battery.

BRIEF DESCRIPTION OF DRAWINGS

To describe the technical solutions in the embodiments of thisapplication more clearly, the following briefly describes theaccompanying drawings required for describing the embodiments of thisapplication. Apparently, the accompanying drawings in the followingdescription show merely some embodiments of this application, and aperson of ordinary skill in the art may still derive other drawings fromthe accompanying drawings without creative efforts.

FIG. 1 is a scanning electron microscope (SEM) image of anegative-electrode active material at a magnification of 1000 timesaccording to an embodiment of this application;

FIG. 2 is a scanning electron microscope (SEM) image of anegative-electrode active material at a magnification of 5000 timesaccording to another embodiment of this application;

FIG. 3 is an ion polishing cross-sectional topography (CP) image of anegative-electrode plate at a magnification of 5000 times after anegative-electrode active material of this application is made into thenegative-electrode plate;

FIG. 4 is a transmission electron microscope (TEM) image of anegative-electrode active material at a magnification of 60000 timesaccording to an embodiment of this application;

FIG. 5 is a schematic diagram of a secondary battery according to anembodiment;

FIG. 6 is an exploded view of FIG. 5 ;

FIG. 7 is a schematic diagram of a battery module according to anembodiment;

FIG. 8 is a schematic diagram of a battery pack according to anembodiment;

FIG. 9 is an exploded view of FIG. 8 ; and

FIG. 10 is a schematic diagram of an apparatus using a secondary batteryas a power source according to an embodiment.

DETAILED DESCRIPTION

In order to make the objectives, technical solutions and beneficialtechnical effects of this application clearer, the following furtherdescribes this application in detail with reference to the embodiments.It should be understood that the embodiments described in thisspecification are merely intended to interpret this application, but notintended to limit this application.

For simplicity, only some numerical ranges are expressly disclosed inthis specification. However, any lower limit may be combined with anyupper limit to form a range not expressly recorded; any lower limit maybe combined with any other lower limit to form a range not expresslyrecorded; and any upper limit may be combined with any other upper limitto form a range not expressly recorded. In addition, although notexpressly recorded, each point or individual value between endpoints ofa range is included in the range. Therefore, each point or individualvalue may act as its own lower limit or upper limit to be combined withany other point or individual value or combined with any other lowerlimit or upper limit to form a range not expressly recorded.

In the description of this specification, it should be noted that,unless otherwise stated, “above” and “below” means inclusion of thenumber itself, and “more” in “one or more” means at least two.

In the description of this specification, unless otherwise stated, theterm “or” indicates inclusion. For example, a phrase “A or B” means “A,B, or both A and B”. More specifically, any one of the followingconditions satisfies the condition “A or B”: A is true (or present) andB is false (or not present); A is false (or not present) and B is true(or present); or both A and B are true (or present).

The foregoing invention content of this application is not intended todescribe each of the disclosed embodiments or implementations of thisapplication. The following description illustrates exemplary embodimentsin more detail by using examples. Throughout this application, guidanceis provided by using a series of embodiments and the embodiments may beused in various combinations. In each instance, enumeration is onlyrepresentative but should not be interpreted as exhaustive.

A secondary battery, also referred to as a rechargeable battery or astorage battery, is a battery that can be charged after being dischargedto activate active materials for continuous use.

Generally, a secondary battery includes a positive-electrode plate, anegative-electrode plate, a separator, and an electrolyte. Duringcharging and discharging of the battery, active ions (for example,lithium ions) are intercalated and deintercalated back and forth betweenthe positive-electrode plate and the negative-electrode plate. Theseparator is provided between the positive-electrode plate and thenegative-electrode plate, mainly plays a role of preventingshort-circuits of the positive and negative electrodes, and allows ionsto pass through. The electrolyte is between the positive-electrode plateand the negative-electrode plate, and mainly plays a role of conductingions.

The inventor has found that a key to improving the fast-chargingcapability of the secondary battery is to improve kinetic performance ofthe negative electrode. The negative-electrode plate generally includesa negative-electrode current collector and a negative-electrode filmlayer, and the negative-electrode film layer includes anegative-electrode active material. The negative-electrode activematerial is usually a material that participates in intercalation anddeintercalation of active ions in the negative-electrode plate duringcharging and discharging of the battery. At present, in order to improvekinetic performance of the battery, most methods are intended to reducea thickness of the negative-electrode film layer or reduce a compacteddensity of the negative-electrode film layer. However, it has beenproved through a lot of researches that in the foregoing methods forimproving battery kinetics, only kinetics of the battery actually in alow SOC state (that is, in an initial charging phase) is improved tosome extent while improvement on kinetics performance of the batteryactually in a high SOC state (that is, in a later charging phase) isrelatively slight. Therefore, the fast-charging capability of thesecondary battery cannot be effectively improved. In addition, theenergy density of the battery is also significantly decreased.

In the preparation process of the negative-electrode active material,the industry usually pays attention to a volume-based median particlesize D_(v)50 thereof. However, because the number of particles with acumulative volume distribution percentage of above 99% in terms of smallparticles is very small, D_(v)99 of the negative-electrode activematerial has been considered unimportant and neglected by the industry.However, the inventor has surprisingly found during in-depth researchthat when D_(v)99 of the negative-electrode active material iscontrolled within a specific range, a fast intercalation capability ofthe negative-electrode active material in a high lithium intercalationstate (corresponding to the high SOC state of the battery) can besignificantly improved. Therefore, the foregoing bottleneck can bebroken, and the fast-charging capability of the secondary battery in thehigh SOC state can be improved.

To this end, this application provides a negative-electrode activematerial, which includes a core and a coating layer covering at leastpart of a surface of the core, where the core includes artificialgraphite, the coating layer includes amorphous carbon, a volume-basedparticle size distribution of the negative-electrode active materialsatisfies D_(v)99≤24 μm, and a volume-based median particle size D_(v)50satisfies 8 μm≤D_(v)50≤15 μm.

The inventor has found through a lot of researches that, when thenegative-electrode active material of this application is used, thebattery can have higher active ion solid-phase diffusion performanceunder the premise of higher energy density. Even in the high lithiumintercalation state during the later charging phase (in the high SOCstate), particles of the negative-electrode active material can stillmaintain good electrochemical reactivity, and active ions can be quicklyintercalated into bulk of the negative-electrode active material andmigrate rapidly, thereby effectively improving a diffusion speed of theactive ions in the negative-electrode plate and also reducing ohmic andconcentration polarization. In this way, an overall charging speed andcharging depth of the negative-electrode plate are greatly increased.Therefore, in this application, high-rate charging of the battery in allSOC states can be implemented, significantly improving the fast-chargingcapability. In addition, migration performance of active ions betweenthe positive and negative electrodes in the battery is relatively goodand features relatively low polarization, significantly improving thecycling performance. Generally, the low SOC state means being below 30%SOC, and the high SOC state means being above 60% SOC.

The artificial graphite described in this application is usually agraphite material obtained through graphitization at a high temperature,and has a relatively high graphitization and crystallization degree.

The amorphous carbon described in this application is typically a carbonmaterial with a lower graphitization and crystallization degree.

Generally, a lattice structure of the artificial graphite tends to be alayered arrangement in a long-range ordered manner; and a latticestructure of the amorphous carbon tends to be disordered. The latticearrangement can be usually observed by using transmission electronmicroscopy (TEM) images.

In some implementations, a shape of the core may be one or more of ablock, a sheet, and an approximate sphere.

In some implementations, a thickness of the coating layer is ≥2 nm,optionally, 2 nm-20 nm; for example, 2 nm-15 nm, 2 nm-10 nm, or 5 nm-10nm.

In some implementations, a coverage of the coating layer on a surface ofthe core is ≥50%, optionally, 60%-100%.

In some implementations, the negative-electrode active material maysatisfy D_(v)99≤23.8 μm, ≤23.5 μm, ≤23 μm, ≤22.5 μm, ≤22 μm, ≤21 μm, or≤20 μm.

In some implementations, the negative-electrode active material cansatisfy D_(v)99≥15 μm, ≥16 μm, ≥17 μm, ≥18 μm, or ≥19 μm.

In some implementations, the negative-electrode active material maysatisfy: 15 μm≤D_(v)99≤24 μm; for example, 16 μm≤D_(v)99≤22 μm, 17μm≤D_(v)99≤24 μm, 17 μm≤D_(v)99≤23 μm, 15 μm≤D_(v)99≤21 μm, 18μm≤D_(v)99≤21 μm, 19 μm≤D_(v)99≤21 μm, 19 μm D_(v)99≤22 μm, 19μm≤D_(v)99≤23 μm, 20 μm≤D_(v)99≤22 μm, 20 μm≤D_(v)99≤21.5 μm, or 20μm≤D_(v)99≤21 μm.

The negative-electrode active material with D_(v)99 being within anappropriate range can further improve a solid-phase diffusion speed ofactive ions of the negative electrode in a high lithium intercalationstate and reduce polarization. This further helps reduce relativelysmall particles therein, and more active ions can be intercalated intothe particles. In addition, a smooth pore channel structure is formed inthe negative-electrode film layer, and a liquid phase conduction path isshortened, thereby further improving the fast-charging capability andcycling performance of the battery. The negative-electrode activematerial with D_(v)99 being within an appropriate range enables thesecondary battery to have both higher fast-charging capability andbetter cycling performance.

In some implementations, the negative-electrode active material cansatisfy D_(v)50≤14 μm, ≤13 μm, or ≤12 μm. Optionally, the D_(v)50 of thenegative-electrode active material is ≥8 μm, ≥9 μm, ≥10 μm, or ≥11 μm.For example, the negative-electrode active material may satisfy 8μm≤D_(v)50≤14 μm, 9 μm≤D_(v)50≤13 μm, 10 μm≤D_(v)50≤14 μm, 12μm≤D_(v)50≤14 μm, 12 μm≤D_(v)50≤13 μm, or 11 μm≤D_(v)50≤13 μm.

The negative-electrode active material with D_(v)50 being within anappropriate range can further shorten a migration path of active ions inparticles of the negative-electrode active material, and helps form asmooth pore structure in the negative-electrode film layer. In this way,the negative-electrode plate has a good solid-phase diffusion speed ofactive ions and good liquid-phase transmission performance, therebyfurther improving the fast-charging capability of the battery. Inaddition, with the negative-electrode active material with D_(v)50 beingwithin an appropriate range, it can be further ensured that thenegative-electrode active material has a higher gram capacity, so thatthe battery obtains a higher energy density and side reaction of theelectrolyte in the negative electrode can be further reduced, therebyimproving the cycling performance of the battery.

In some implementations, a particle size uniformity of thenegative-electrode active material is 0.25-0.45, for example, may be0.28-0.4, 0.32-0.4, 0.32-0.38, 0.30-0.36, 0.31-0.35, or 0.32-0.36. Theparticle size uniformity of the negative-electrode active material maycharacterize a dispersion degree by which the particle size of allparticles of the negative-electrode active material deviates from thevolume-based median particle size D_(v)50 of the negative-electrodeactive material, which reflects uniformity of particle size distributionof the negative-electrode active material. When the particle sizeuniformity of the negative-electrode active material is within theforegoing range, a relatively short liquid-phase transmission path islikely to be formed in the negative-electrode film layer, and a largecontact area is present between the particles. This is good for electronconduction and active ion transmission in the negative-electrode plate,further improving the fast-charging capability of the battery. Inaddition, close contact between the particles of the negative-electrodefilm layer can be implemented, and the negative-electrode plate can havea higher compacted density, thereby improving the energy density of thebattery.

In some implementations, the negative-electrode active materialsatisfies: 0.6≤(D_(v)90−D_(v)10)/D_(v)50≤1.8. For example,(D_(v)90−D_(v)10)/D_(v)50 of the negative-electrode active material is0.8 to 1.4, 0.9 to 1.3, 1.0 to 1.25, or 1.2 to 1.6.(D_(v)90−D_(v)10)/D_(v)50 of the negative-electrode active materialreflects a degree by which the particle size of larger particles and theparticle size of smaller particles deviate from the volume-based medianparticle size D_(v)50 in the negative-electrode active material. Thenegative-electrode active material with (D_(v)90−D_(v)10)/D_(v)50 beingwithin an appropriate range helps improve processing performance of anegative-electrode slurry and the negative-electrode film layer, so thatthe negative-electrode film layer has relatively high particledistribution uniformity. In this way, different areas of thenegative-electrode film layer all exhibit relatively high active iontransmission performance, and the fast-charging capability of thebattery is further improved.

In some implementations, the volume-based particle size distributionD_(v)90 of the negative-electrode active material is 13 μm-18 μm, forexample, may be 13 μm-16 μm, 14 μm-17 μm, or 15 μm-18 μm. Thenegative-electrode active material with D_(v)90 being within anappropriate range can further improve the solid-phase diffusion speed ofactive ions in the negative-electrode film layer, thereby furtherimproving the fast-charging capability of the battery. In addition, thenegative-electrode active material can further have a relatively highgram capacity, which helps increase the energy density of the battery.

In some implementations, the volume-based particle size distributionD_(v)10 of the negative-electrode active material is 5 μm-10 μm, forexample, may be 6 μm-8 μm. The content of small particles in thenegative-electrode active material is small, which can reduce sidereaction between the electrolyte and the material, and improve thecycling performance and storage performance of the battery.

In some implementations, a particle size specific surface area of thenegative-electrode active material is 0.4 μm²/g-0.75 μm²/g, for example,may be 0.4 μm²/g-0.7 μm²/g, 0.42 μm²/g-0.68 μm²/g, 0.46 μm²/g-0.55μm²/g, 0.5 μm²/g-0.68 μm²/g, or 0.5 μm²/g-0.65 μm²/g.

It should be noted that the “particle size specific surface area” of thenegative-electrode active material of this application is not the sameas a conventional “specific surface area” of the negative-electrodeactive material. At present, the specific surface area (SSA) of thenegative-electrode active material in the industry is mostly obtained byusing a gas adsorption BET method, and is merely used to characterize aphysical adsorption specific surface area of the negative-electrodeactive material. The “particle size specific surface area” of thenegative-electrode active material in this application is obtained byusing a laser diffraction particle size analysis method, and may be usedto characterize a degree by which a profile of the negative-electrodeactive material deviates from a sphere.

The inventor has found that the negative-electrode active material withthe particle size specific surface area being within an appropriaterange can increase ion deintercalation channels in thenegative-electrode film layer and reduce a charge exchange resistance,and enables the negative-electrode film layer to form smoother porechannels, so as to improve wettability of the electrolyte. This furtherimproves a solid-phase and liquid-phase transmission speed of activeions in the negative-electrode plate, and further improves the fastcharging performance and cycling performance of the battery. Inaddition, the negative-electrode active material with the particle sizespecific surface area being within an appropriate range can also improvethe compacted density of the negative-electrode film layer, therebyincreasing the energy density of the battery.

In some implementations, as shown in FIG. 1 and FIG. 2 , secondaryparticles are included in the negative-electrode active material.Optionally, the quantity proportion of the secondary particles in thenegative-electrode active material is ≥50%. For example, the quantityproportion of the secondary particles in the negative-electrode activematerial is 50%-100%, 60%-100%, 60%-90%, 70%-100%, 70%-95%, 70%-90%,70%-80%, or 75%-85%. When the negative-electrode active materialcontains a large quantity of secondary particles, the active-iondeintercalation channels in the negative-electrode film layer increase,thereby further improving the fast-charging capability of the battery,reducing polarization, and improving the cycling performance. Inparticular, when the negative-electrode active material includes bothsecondary particles and primary particles, side reactions of theelectrolyte in the negative electrode can be reduced, and the cyclingperformance and storage performance of the battery can be furtherimproved.

In some implementations, the graphitization degree of thenegative-electrode active material is 91.0%-96.0%, for example, may be94.0%-95.0%, or 93.0%-94.5%. The negative-electrode active material withthe graphitization degree being within the foregoing range can have alarger interlayer spacing in the particle structure, and has a lowerpowder resistance, further improving the fast-charging capability.

In some implementations, the gram capacity of the negative-electrodeactive material is 345 mAh/g-360 mAh/g, for example, may be 350mAh/g-358 mAh/g, 351 mAh/g-356 mAh/g, or 352 mAh/g-355 mAh/g. Thenegative-electrode active material with a higher gram capacity canincrease the energy density of the battery. The gram capacity of thenegative-electrode active material being within the foregoing rangemeans that the active-ion migration path of the material is short. Thiscan improve the fast-charging capability of the battery.

In some implementations, a tap density of the negative-electrode activematerial is 0.9 g/cm³-1.3 g/cm³, for example, may be 1.0 g/cm³-1.1g/cm³. The negative-electrode active material with the tap density beingwithin the given range can make the particles of the negative-electrodefilm layer be in good contact, improving the fast-charging capability ofthe battery. In addition, the particles are tightly packed, which canfurther increase the energy density of the battery.

In some implementations, a powder compacted density of thenegative-electrode active material under 2 kN pressure is 1.55g/cm³-1.67 g/cm³; and for example, may be 1.60 g/cm³-1.65 g/cm³. Thenegative-electrode active material with the powder compacted densityunder the pressure of 2 kN being within the given range can implementclose contact between the particles of the negative-electrode film layerto form good electrolyte infiltration pore channels, thereby improvingthe fast-charging capability and cycling performance of the battery.

In this application, D_(v)99, D_(v)90, D_(v)50, D_(v)10, the particlesize uniformity, and the particle size specific surface area of thenegative-electrode active material can be measured by using the laserdiffraction particle size analysis method. For example, referring to thestandard GB/T 19077-2016, a laser particle size analyzer (for example,Malvern Master Size 3000) is used for measurement.

D_(v)99 is a particle size corresponding to a cumulative volumedistribution percentage of the negative-electrode active materialreaching 99%; D_(v)90 is a particle size corresponding to a cumulativevolume distribution percentage of the negative-electrode active materialreaching 90%; D_(v)50 is a particle size corresponding to a cumulativevolume distribution percentage of the negative-electrode active materialreaching 50%; and D_(v)10 is a particle size corresponding to acumulative volume distribution percentage of the negative-electrodeactive material reaching 10%.

In this application, the negative-electrode active material can be madeinto a negative-electrode plate, and an ion polishing cross-sectiontopography (CP) test is conducted on the negative-electrode plate toobserve a material type of the core. As an example, the test method maybe as follows: cutting a prepared negative-electrode plate into ato-be-tested sample of a specific size (for example, 2 cm×2 cm), andfastening the negative-electrode plate to a sample stage by usingparaffin; placing the sample stage in a sample holder and locking thesample holder firmly, turning on an argon ion cross-section polisher(for example, IB-19500CP) and performing evacuation (for example, 10-4Pa), setting argon flow (for example, 0.15 MPa), voltage (for example, 8KV), and polishing time (for example, 2 hours), and then adjusting thesample stage to rocking mode to start polishing. For the sample test,refer to JY/T010-1996. A region of the to-be-tested sample may berandomly selected for scanning and testing, and an ion polishingcross-sectional topography (CP) image of the negative-electrode plate isobtained at a magnification (for example, 5000 times). For example, itcan be seen from FIG. 3 of this application that the core of thenegative-electrode active material of this application is artificialgraphite.

In this application, the structure of the negative-electrode activematerial (for example, the core and the coating layer) can be tested byusing a device and method known in the art. As an example, the followingsteps may be performed: selecting a microgrid with a specific diameter(for example, 3 mm in diameter), holding an edge of the microgrid withpointed tweezers, placing a membrane surface of the microgrid upward (toobserve a shiny side, namely the membrane surface, under light), andgently placing it flat on white filter paper; adding an appropriateamount of graphite particle sample (for example, 1 g) to a beakercontaining an appropriate amount of ethanol, and performing ultrasonicvibration for 10 min to 30 min; sucking with a glass capillary andadding 2-3 drops of the to-be-tested sample on the microgrid; and afterbaking in the oven for 5 min, placing the microgrid with theto-be-tested sample on the sample stage, and conducting testing at aspecific magnification (for example, 60,000 times) by using atransmission electron microscope (for example, Hitachi HF-3300SCs-corrected STEM), so as to obtain a transmission electron microscope(TEM) image of the to-be-tested sample. For example, it can be seen fromFIG. 4 of this application that the negative-electrode active materialof this application includes the core and the coating layer.

In this application, the primary particle and secondary particle havemeanings known in the art. The primary particle is a non-agglomeratedparticle, and the secondary particle is an agglomerated particle formedthrough aggregation of two or more primary particles. The primaryparticles and secondary particles can be easily distinguished with SEMimages photographed by using a scanning electron microscope.

The quantity proportion of the secondary particles in thenegative-electrode active material can be measured by using a methodknown in the art. An example test method is as follows: placing thenegative-electrode active material flat and attaching it onto aconductive adhesive to obtain a to-be-tested sample 6 cm long and 1.1 cmwide; and conducting testing on the morphology of the particles by usinga scanning electron microscope (for example, ZEISS Sigma 300). Fortesting, refer to JY/T010-1996. In order to ensure accuracy of testresults, a plurality (for example, 5) of different regions of theto-be-tested sample may be randomly selected for scanning and testing,and a quantity proportion of the secondary particles in each region inthe total number of particles is obtained through calculation at amagnification (for example, 1000 times), which is a quantity proportionof the secondary particles in the area. An average value of test resultsof the plurality of test regions is used as the quantity proportion ofthe secondary particles in the negative-electrode active material. Inorder to ensure accuracy of the test results, a plurality (for example,10) of test samples may be used to repeat the foregoing test, and anaverage value of test results of all the test samples is used as thefinal test result.

The graphitization degree of the negative-electrode active material hasa meaning known in the art, and can be tested by using a method known inthe art. For example, an X-ray diffractometer (for example, Bruker D8Discover) may be used. For testing, refer to JIS K 0131-1996 and JB/T4220-2011. A size of d₀₀₂ is measured and then the graphitization degreeis calculated according to a formula G=(0.344-d₀₀₂)/(0.344-0.3354)×100%,where d₀₀₂ is an interlayer spacing of the graphite crystal structure innanometers (nm). In the X-ray diffraction analysis test, a copper targetmay be used as an anode target, CuKα rays are used as a radiationsource, with a ray wavelength λ=1.5418 Å and a scanning 2θ angle rangeof 20°-80°, and a scanning rate may be 4°/min.

The tap density of the negative-electrode active material has a meaningknown in the art, and can be tested by using a method known in the art.For example, reference may be made to the standard GB/T 5162-2006, and apowder tap density tester may be used for testing. For example, theFZS4-4B type tap density meter manufactured by Beijing Iron and SteelResearch Institute is used, and test parameters are as follows:vibration frequency: 250±15 times/min, amplitude: 3±0.2 mm, vibrationfrequency: 5000 times, and measuring cylinder: 25 mL.

The powder compacted density of the negative-electrode active materialunder a pressure of 2 kN has a meaning known in the art, and can betested by using a method known in the art. For example, reference ismade to the standard GB/T24533-2009, and an electronic pressure testingmachine (for example, UTM7305) is used for testing. An example testmethod is as follows: weighing and adding 1 g of the negative-electrodeactive material to a mold with a bottom area of 1.327 cm², applyingpressure to 200 kg (equivalent to 2 kN) and holding the pressure for 30s, releasing the pressure and waiting for 10 s, and then recording andcalculating the powder compacted density of the negative-electrodeactive material under the 2 kN pressure.

The gram capacity of the negative-electrode active material has ameaning known in the art, and can be tested by using a method known inthe art. An example test method is as follows: mixing the preparednegative-electrode active material, a conductive agent carbon black(Super P), and a binder polyvinylidene fluoride (PVDF) in a mass ratioof 91.6:1.8:6.6 evenly in a solvent N-methylpyrrolidone (NMP) to obtaina slurry, applying the prepared slurry on a copper foil currentcollector, and drying it in an oven for later use. A metal lithium sheetis used as a counter electrode, and a polyethylene (PE) film is used asthe separator. Ethylene carbonate (EC), ethyl methyl carbonate (EMC),and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1:1, andthen LiPF₆ was uniformly dissolved in the foregoing solution to obtainan electrolyte, where the concentration of LiPF₆ is 1 mol/L. A CR2430coin battery is assembled in an argon gas-protected glove box. Afterbeing left standing for 12 hours, the obtained coin battery isdischarged to 0.005V at a constant current of 0.05C at 25° C. and leftstanding for 10 minutes; discharged to 0.005V at a constant current of50 μA, left standing for 10 minutes, and discharged to 0.005V at aconstant current of 10 μA; and then charged to 2V at a constant currentof 0.1C. A-charging capability is recorded. A ratio of the-chargingcapability to the mass of the negative-electrode active material is thegram capacity of the prepared negative-electrode active material.

It should be noted that, for testing of the foregoing parameters of thenegative-electrode active material, a negative-electrode active materialsample may be directly used for testing, or a sample for testing may beobtained from the secondary battery.

In a case that the foregoing testing samples are fetched from thesecondary battery, as an example, a sample may be obtained by performingthe following steps:

-   -   (1) discharging the secondary battery (for sake of safety,        generally making the battery in fully discharged state),        disassembling the battery, and taking out the negative-electrode        plate, using dimethyl carbonate (DMC) to soak the        negative-electrode plate for a specific time (for example, 2-10        hours); and then taking out the negative-electrode plate and        drying it at a given temperature for a specific time (for        example, 60° C., 4 h), and taking out the negative-electrode        plate obtained after drying;    -   (2) baking, at a given temperature for a specific time (for        example, 400° C., 2 h), the negative-electrode plate obtained        after drying in step (1), randomly selecting a region from the        negative-electrode plate obtained after baking, and obtaining a        negative-electrode active material sample (the sample may be        obtained by scraping powder using a blade); and    -   (3) sieving the negative-electrode active material collected in        step (2) (for example sieving with a 200-mesh screen), to        finally obtain the negative-electrode active material sample        that can be used for testing the foregoing material parameters        of this application.

This application provides a preparation method of the negative-electrodeactive material below, and the negative-electrode active material can beobtained by using the preparation method. The preparation method of thenegative-electrode active material may include the following steps (A)and (B).

-   -   (A) providing a core, where the core includes artificial        graphite; and    -   (B) coating the core to form a coating layer on at least part of        a surface of the core, so as to obtain a negative-electrode        active material, where the coating layer includes amorphous        carbon, and the negative-electrode active material satisfies        D_(v)99≤24 μm and 8 μm≤D_(v)50≤15 μm.

In some implementations, in step (A), the preparation method ofartificial graphite may include steps (a) to (d).

-   -   (a) providing a coke raw material;    -   (b) performing shaping processing on the coke raw material to        obtain a precursor;    -   (c) granulating the precursor to obtain a granulated product;        and    -   (d) graphitizing the granulated product to obtain artificial        graphite, where a volume-based median particle size D_(v)50 of        the artificial graphite is 6 μm-14 μm, and a volume-based        particle size distribution D_(v)99 is 17 μm-26 μm.

In some implementations, D_(v)50 and D_(v)99 of the coke raw materialcan be adjusted, so that D_(v)50 of the coke raw material is 7 μm-12 μm,and D_(v)99 is 15 μm-21 μm. The coke raw material with D_(v)50 andD_(v)99 being within the given ranges helps improve subsequent shapingand granulation processes, so that the final negative-electrode activematerial has an appropriate secondary particle content and appropriateD_(v)50 and D_(v)99. Optionally, D_(v)50 of the coke raw material is 8μm-12 μm, 8 μm-11.5 μm, or 9 μm-11 μm. Optionally, D_(v)99 of the cokeraw material is 16 μm-21 μm, 17 μm-21 μm, 17 μm-20 μm, or 17 μm-19 μm.

In step (a), the coke raw material can be directly obtainedcommercially, or be obtained by pulverizing the coke material. In someimplementations, the coke material may be pulverized to control D_(v)50and D_(v)99 of the coke raw material within desired ranges. The cokematerial may be pulverized by using a device and method known in theart, for example, through jet milling, mechanical milling, or rollermilling. A large quantity of extremely small particles are usuallygenerated during pulverization, and sometimes there are also extremelylarge particles. Therefore, after pulverization, classification may beperformed according to requirements, so as to remove extremely smallparticles and extremely large particles from the pulverized powder. Thecoke raw material with desired particle size distribution can beobtained after classification. Classification may be performed by usinga device and method known in the art, for example, a classificationscreen, a gravity classifier, or a centrifugal classifier.

Pulverization of the coke material may be performed at a process stepincluding a pulverizer, a classifier and an induced draft fan. Duringpulverization, D_(v)50 and D_(v)99 of the obtained coke raw materialscan be controlled within desired ranges by adjusting feeding frequency,pulverizing frequency, classification frequency, and induced draftingfrequency. Compared with relatively low classification frequency in theconventional pulverization process, the method in this application canincrease the classification frequency, which helps remove excessivelysmall particles. Compared with relatively high induced draftingfrequency in the conventional pulverization process, the method in thisapplication can increase the induced drafting frequency, which helpsremove excessively large particles. In addition, compared with theconventional pulverization process that controls the frequency within alarger range, the method in this application can also control frequencyof the main engine, classification frequency, and induced draftingfrequency within a smaller frequency range, thereby reducing particlesize distribution of the coke raw material. For example, D_(v)50 andD_(v)99 of the coke raw material are controlled within smaller ranges.The feeding frequency can also be adjusted to control a feeding amount,further improving pulverization effects of the material.

In some implementations, the feeding frequency may be 10 Hz-40 Hz, forexample, 25 Hz-35 Hz.

In some implementations, the pulverization frequency may be 20 Hz-50 Hz,for example, 35 Hz-45 Hz.

In some implementations, the classification frequency may be 20 Hz-50Hz, for example, 40 Hz-50 Hz.

In some implementations, the induced drafting frequency may be 30 Hz-55Hz, for example, 35 Hz-45 Hz.

Those skilled in the art may choose to adjust one or more of theforegoing process conditions according to actual operation conditions,and finally obtain the coke raw material with D_(v)50 of 7 μm-12 μm andD_(v)99 of 15 μm-21 μm.

In some implementations, the coke raw material in step (a) includes oneor more of petroleum-based non-needle coke and petroleum-based needlecoke. Optionally, the coke raw material includes petroleum green coke.

In some implementations, the volatile content C₁ of the coke rawmaterial in step (a) satisfies 1%≤C₁≤12%. Optionally, the volatilecontent C₁ of the coke raw material is 3%-10%, 5%-9%, 6%-8%, 7%-8.5%,7.5%-8.5%, or the like. The coke raw material with the volatile contentbeing within an appropriate range helps improve the particle sizedistribution of the material in the subsequent granulation process, sothat the negative-electrode active material has desired particle sizedistribution. In addition, the coke raw material with the volatilecontent being within an appropriate range can also make the preparedartificial graphite have higher structural strength, and increase thecycle life of the negative-electrode active material, thereby improvingthe cycling performance of the battery.

The volatile content of the coke raw material can be tested by using amethod known in the art. For example, refer to SH/T 0026-1990 fortesting.

In step (b), edges and corners of particles of the coke raw material canbe polished through shaping, which is conducive to the subsequentgranulation process, so that the secondary particles in the resultingnegative-electrode active material have higher structural stability.Shaping treatment on the coke raw material may be performed by using adevice and method known in the art, such as a shaping machine or othershaping devices.

In some implementations, after shaping treatment of the coke rawmaterial, classification is further performed, so that the precursor hasD_(v)50 of 8 μm-13 μm and D_(v)99 of 16 μm-22 μm. In this way, thenegative-electrode active material obtained finally has an appropriatecontent of the secondary particles and has appropriate D_(v)50 andD_(v)99. Optionally, D_(v)50 of the precursor is 9 μm-12 μm, 9 μm-11 μm,10 μm-12 μm, or 10 μm-11 μm. D_(v)99 of the precursor is 17 μm-22 μm, 18μm-21 μm, or 18 μm-20 μm. Classification may be performed by using adevice and method known in the art, for example, a classificationscreen, a gravity classifier, or a centrifugal classifier.

Shaping and classification can be performed at a process step includinga shaping machine, a classification machine, and an induced draft fan.During shaping and classification, D_(v)50 and D_(v)99 of the obtainedprecursor can be controlled within a required range by adjusting shapingfrequency (for example, main-machine frequency and auxiliary-machinefrequency of the shaping machine), classification frequency, and induceddrafting frequency. The inventor has found that, compared with aconventional shaping and classification process, the method of thisapplication improves the shaping frequency during processing,appropriately prolongs a shaping time, and also reduces theclassification frequency and induced drafting frequency duringprocessing, so that D_(v)50 and D_(v)99 of the obtained precursor arecontrolled within a target range.

The obtained precursor may also have an appropriate particle sizeuniformity (U₁), which helps improve the particle size uniformity of theobtained negative-electrode active material.

In some implementations, the particle size uniformity U₁ of theprecursor satisfies 0.2≤U₁≤0.55, for example, 0.2≤U₁≤0.5, 0.25≤U₁≤0.45,0.3≤U₁≤0.45, 0.3≤U₁≤0.4, 0.35≤U₁≤0.55, or 0.35≤U₁≤0.45.

In some implementations, in step (b), shaping and classification areperformed on the coke raw material based on the following values of theshaping machine: a main-machine frequency of 35 Hz-40 Hz, anauxiliary-machine frequency of 60 Hz-70 Hz, a classification frequencyof 40 Hz-50 Hz, an induced drafting frequency of 10 Hz-25 Hz, and ashaping time of 160-180 s, so as to obtain the precursor with D_(v)50 of8 μm-13 μm and D_(v)99 of 16 μm-22 μm.

In step (c), the precursor is granulated to form secondary particles byagglomerating independently dispersed primary particles. As a result,isotropy of the artificial graphite is improved, and active ions can beintercalated into the particles from all directions, thereby improving asolid-phase lithium intercalation rate and reducing polarization.

In some implementations, D_(v)50 of the granulated product obtained instep (c) may be 9 μm-15 μm, and D_(v)99 is 17 μm-24 μm. Optionally,D_(v)50 of the granulated product is 10 μm-14 μm, 11 μm-15 μm, or 11μm-13 μm. Optionally, D_(v)99 of the granulated product is 18 μm-24 μm,or 19 μm-22 μm. The granulated product with D_(v)50 and D_(v)99 beingwithin appropriate ranges makes D_(v)50 and D_(v)99 of the finallyobtained negative-electrode active material fall within a desiredranges.

Granulation may be performed in step (c) by using a device known in theart, for example, a granulator. The granulator usually includes anagitating reactor and a temperature control module for the reactor. Thegranulation degree can be adjusted by adjusting a stirring speed,heating rate, granulation temperature, cooling rate, and the like in thegranulation process, and D_(v)50 and D_(v)99 of the resulting granulatedproduct can also be controlled within the desired ranges. Further,through adjustment in the foregoing granulation process, D_(v)10 andD_(v)90 of the resulting granulated product can also be controlledwithin the required ranges, so that D_(v)10 and D_(v)90 of the finallyobtained negative-electrode active material can meet the requirements.

In some implementations, the precursor can be mixed with a binder, andthen granulated at a high temperature. A mixing temperature may be 20°C. to 40° C. Compared with a preparation process of conventionalgraphite, this application appropriately increases mixing frequency andshortens a mixing time, thereby improving the granulation degree andhelping control D_(v)50 and D_(v)99 of the resulting granulated productwithin required ranges.

A temperature for high-temperature granulation may be determined basedon a type of the binder. The binder is softened at a high temperature tobond particles, implementing granulation. In some implementations, thebinder is asphalt. In these embodiments, the granulation temperature mayrange from 700° C. to 800° C. This application further improves aheating program of the high-temperature granulation process, and usesstepwise heating. A plurality (for example, 2 to 4) of programmedheating platforms are set in the heating process, so that the granulatedproduct can obtain the desired particle size distribution. In addition,the particle size uniformity of the granulated product is relativelygood, so that the subsequent artificial graphite and the finalnegative-electrode active material product can obtain a better particlesize uniformity.

In some implementations, in step (c), the mixing frequency can becontrolled to be 35 Hz-38 Hz, and the mixing time is 50 min-65 min. Thetemperature is increased to 300° C.-400° C. at 6-10° C./min andmaintained at the temperature for 1 h-2 h; increased to 500° C.-600° C.at 6-10° C./min and maintained at the temperature for 1 h-2 h; increasedto 700° C.-800° C. at 6-10° C./min and maintained at the temperature for1 h-2 h; and then is naturally cooled, so as to obtain a granulatedproduct.

In some implementations, in step (c), an amount C₂ of the binder that isadded during granulation, the volatile content C₁ of the coke rawmaterial, and the particle size uniformity U₁ of the precursor satisfy:21%≤(C₁+C₂)/U₁×100% 50%. Optionally, 25%≤(C₁+C₂)/U₁×100%≤45%,25%≤(C₁+C₂)/U₁×100%≤38%, 27%≤(C₁+C₂)/U₁×100%≤38%,30%≤(C₁+C₂)/U₁×100%≤40%, or 31%≤(C₁+C₂)/U₁×100%≤35%. When the amount C₂of the binder that is added during granulation, the volatile content C₁of the coke raw material, and the particle size uniformity U₁ of theprecursor satisfy the foregoing relationship, the granulation degree ofparticles of the negative-electrode active material can be improved, andthe ion deintercalation performance and structural stability of thenegative-electrode active material can be improved.

The amount C₂ of the binder added during granulation is a percentage ofthe weight of the binder added during granulation in the total weight ofthe precursor. The granulation process is performed under a conditionwith the binder added or not added.

In some implementations, the amount of the binder C₂ added duringgranulation may satisfy 0%≤C₂≤16%, optionally, 1%≤C₂≤12%, 2%≤C₂≤10%,4%≤C₂≤7%, or 5%≤C₂≤9%.

In some implementations, in step (d), graphitization is performed on thegranulated product at a temperature of 2800° C. to 3200° C., so as toobtain artificial graphite with an appropriate graphitization degree.Optionally, the temperature for graphitization may be 2900° C. to 3100°C.

In step (d), graphitization may be performed by using a device known inthe art, for example, a graphitization furnace, and further for example,an Acheson graphitization furnace. After graphitization is completed, asmall quantity of extremely large particles formed by agglomeration inthe high-temperature graphitization process may be removed from thegranulated product through sieving, so that D_(v)50 and D_(v)99 of thefinal negative-electrode active material fall within the requiredranges.

In some implementations, D_(v)50 of the artificial graphite obtained instep (d) may be 6.5 μm-14 μm, 7 μm-14 μm, 6 μm-13 μm, 7 μm-13.5 μm, 8μm-12 μm, 9 μm-12 μm, 9 μm-11 μm, 10 μm-13 μm, 10 μm-12 μm, 6.5 μm-12μm, or 6.5 μm-12.5 μm.

In some implementations, D_(v)99 of the artificial graphite obtained instep (d) may be 18 μm-24 μm, 19 μm-26 μm, 21 μm-26 μm, 20 μm-25 μm, 20μm-23 μm, or 19.5 μm-22 μm.

In some implementations, the particle size uniformity U₂ of theartificial graphite obtained in step (d) may satisfy 0.22≤U₂≤0.48,optionally, 0.25≤U₂≤0.45, 0.26≤U₂≤0.43, 0.3≤U₂≤0.4, or 0.33≤U₂≤0.38. Theobtained artificial graphite with the particle size uniformity beingwithin an appropriate range helps make the particle size uniformity ofthe finally obtained negative-electrode active material be within adesired range.

In some implementations, the step (B) may include: (e) coating the corewith an organic carbon source and performing heat treatment to form anamorphous carbon coating layer on at least part of the surface of thecore, so as to obtain the negative-electrode active material.

In some implementations, in step (e), after an amorphous carbon coatinglayer is formed on at least part of the surface of the core, sieving isperformed to obtain the negative-electrode active material.

As an example, the artificial graphite obtained in step (d) may be mixedwith the organic carbon source, so that the organic carbon source coatsat least part of a surface of the artificial graphite. Heating treatmentis then performed at a temperature of 700° C. to 1800° C. to carbonizethe organic carbon source, so as to form an amorphous carbon coatinglayer on at least part of the surface of the artificial graphite.Optionally, the temperature of heating treatment is 1000° C. to 1300° C.

In some implementations, an amount C₃ of the organic carbon source thatis added in the coating process, a volatile content C₁ of the coke rawmaterial, an amount C₂ of the binder, and a particle size uniformity U₂of the artificial graphite satisfy: 20%≤(C₁+C₂+C₃)/U₂×100%≤56%. Theorganic carbon source satisfies 1.2%≤C₃×carbon residue rate ≤2.5%. Theamount C₃ of the organic carbon source is a percentage of a weight ofthe organic carbon source added in the coating process in the totalweight of the artificial graphite. The carbon residue rate is a carbonresidue rate of the organic carbon source, which can be measured byusing an LP-5731 coal pitch coking value tester. For the testing, referto GB/T268 “Test Method for Carbon Residues in Petroleum Products”, andGB/T8727-2008 “Test Method for Coking Values of Coal Pitch Products”.

The amount of the organic carbon source added in the coating processsatisfying the foregoing relationship can improve the granulation degreeof the negative-electrode active material, so that the particle sizeuniformity of negative-electrode active material, the particle sizespecific surface area, and a quantity proportion of the secondaryparticles fall within the foregoing ranges. In addition, when the amountof the organic carbon source falls within the foregoing range, and thecoating layer has an appropriate proportion in the negative-electrodeactive material, the negative-electrode active material can have bothgood kinetic performance and long cycle life. Optionally,30%≤(C₁+C₂+C₃)/U₂×100%≤48%. Further optionally,40%≤(C₁+C₂+C₃)/U₂×100%≤48%. Optionally, 1.5% C₃×carbon residue rate≤2.4%, 1.8%≤C₃×carbon residue rate ≤2.3%, or 2%≤C₃×carbon residue rate≤2.2%.

Optionally, 2%≤C₃≤8%. For example, C₃ may be 3%, 4%, 5%, 6%, or 7%.

In some implementations, the organic carbon source may be selected fromone or more of pitch (for example, coal pitch or petroleum pitch),phenolic resin, coconut shell, and the like, and further optionally, ispitch.

In the foregoing preparation process, the coke raw material usuallycontains some impurity elements (for example, iron, nickel, chromium,zinc, sulfur, or silicon), and some impurity elements (such as iron orcopper) may also be introduced by devices used during pulverization,shaping, the granulation. In normal cases, content of the impurityelements in the core is small, generally less than 1 ppm.

In the foregoing preparation process, the organic carbon source used inthe coating process and the device used for coating may introduce atrace amount of impurity elements in the coating layer.

Secondary Battery

This application further provides a secondary battery. The secondarybattery includes a positive-electrode plate, a negative-electrode plate,and an electrolyte. During charging and discharging of the battery,active ions are intercalated and deintercalated back and forth betweenthe positive-electrode plate and the negative-electrode plate. Theelectrolyte is between the positive-electrode plate and thenegative-electrode plate, and plays a role of conducting ions.

[Negative-Electrode Plate]

In the secondary battery of this application, the negative-electrodeplate includes a negative-electrode current collector and anegative-electrode film layer provided on at least one surface of thenegative-electrode current collector, where the negative-electrode filmlayer includes any one or more of the negative-electrode activematerials in this application.

In some implementations, optionally, in addition to the foregoingnegative-electrode active material of this application, thenegative-electrode film layer may further include a specific quantity ofother commonly used negative-electrode active materials, for example,one or more of natural graphite, other artificial graphite, soft carbon,hard carbon, silicon-based materials, tin-based materials, and lithiumtitanate. The silicon-based material may be selected from one or more ofelemental silicon, silicon oxide, and silicon-carbon composite. Thetin-based material may be selected from one or more of elemental tin,tin-oxygen compound, and tin alloy.

In the secondary battery of this application, the negative-electrodefilm layer usually includes the negative-electrode active material andoptionally a binder, optionally a conductive agent, and other optionalauxiliary agents, and is usually obtained through drying after anegative-electrode slurry is applied. The negative-electrode slurry isusually obtained by dispersing the negative-electrode active materialand optionally a conductive agent, a binder, or the like in a solventand stirring uniformly. The solvent may be N-methylpyrrolidone (NMP) ordeionized water.

As an example, the conductive agent is one or more of superconductingcarbon, carbon black (for example, acetylene black or Ketjen black),carbon dots, carbon nanotube, graphene, and carbon nanofiber.

As an example, the binder may include one or more of styrene-butadienerubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylicresin, polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethylchitosan (CMCS).

Other optional auxiliary agents are, for example, a thickener (forexample, sodium carboxymethyl cellulose CMC-Na) or a PTC thermistormaterial.

In addition, in the secondary battery of this application, thenegative-electrode plate does not exclude additional functional layersother than the negative-electrode film layer. For example, in someimplementations, the negative-electrode plate of this application mayfurther include a conductive primer layer (which is, for example, formedby a conductive agent and a binder) sandwiched between thenegative-electrode current collector and the first negative-electrodefilm layer and disposed on the surface of the negative-electrode currentcollector. In some other implementations, the negative-electrode plateof this application may further include a protective layer covering asurface of a second negative-electrode film layer.

[Positive-Electrode Plate]

In the secondary battery of this application, the positive-electrodeplate includes a positive-electrode current collector and apositive-electrode film layer that is provided on at least one surfaceof the positive-electrode current collector and that includes apositive-electrode active material. For example, the positive-electrodecurrent collector has two surfaces opposite in its thickness direction,and the positive-electrode film layer is provided on either or both ofthe two opposite surfaces of the positive-electrode current collector.

In the secondary battery of this application, a positive-electrodeactive material for the secondary battery known in the art can be usedas the positive-electrode active material. For example, thepositive-electrode active material may include one or more of lithiumtransition metal oxide, olivine-structured lithium-containing phosphate,and respective modified compounds thereof. Examples of the lithiumtransition metal oxide may include, but are not limited to, one or moreof lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide,lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithiumnickel manganese oxide, lithium nickel cobalt manganese oxide, lithiumnickel cobalt aluminum oxide, and modified compounds thereof. Examplesof the olivine-structured lithium-containing phosphate may include, butare not limited to, one or more of lithium iron phosphate, lithium ironphosphate-carbon composite, lithium manganese phosphate, lithiummanganese phosphate-carbon composite, lithium iron manganese phosphate,lithium iron manganese phosphate-carbon composite, and respectivemodified compounds thereof. This application is not limited to thesematerials, and other conventionally known materials that can be used aspositive-electrode active materials for secondary batteries can also beused.

In some optional implementations, in order to further increase theenergy density of the battery, the positive-electrode active materialmay include one or more of the lithium transition metal oxides shown inFormula 1 and modified compounds thereof.

Li_(a)Ni_(b)Co_(c)M_(d)O_(e)A_(f)  Formula 1

In Formula 1, 0.8≤a≤1.2, 0.5≤b<1, 0<c<1, 0<d<1, 1≤e≤2, or 0≤f≤1, where Mis selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti,and B; and A is selected from one or more of N, F, S, and Cl.

In this application, the modified compounds of the foregoing materialsmay be modified by doping or surface coating on the positive-electrodeactive material.

In the secondary battery of this application, the positive-electrodefilm layer usually includes the positive-electrode active material andoptionally a binder, optionally a conductive agent, and is usuallyobtained through drying and cold pressing after a positive-electrodeslurry is applied. The positive-electrode slurry is usually obtained bydispersing the positive-electrode active material and optionally aconductive agent, a binder, or the like in a solvent and stirringuniformly. The solvent may be N-methylpyrrolidone (NMP).

As an example, the binder for the positive-electrode film layer mayinclude one or more of polyvinylidene fluoride (PVDF) andpolytetrafluoroethylene (PTFE).

As an example, the conductive agent for the positive-electrode filmlayer may include one or more of superconducting carbon, carbon black(for example, acetylene black or Ketjen black), carbon dots, carbonnanotube, graphene, and carbon nanofiber.

In the secondary battery of this application, the positive-electrodecurrent collector may use a metal foil or a composite current collector(a metal material may be provided on a polymer matrix to form thecomposite current collector). As an example, the positive-electrodecurrent collector may use an aluminum foil.

[Electrolyte]

The secondary battery of this application has no specific limitation ona type of the electrolyte, which can be selected as required. Forexample, the electrolyte may be selected from at least one of solidelectrolyte and liquid electrolyte (that is, electrolyte).

In some implementations, the electrolyte uses a liquid electrolyte. Theelectrolyte includes an electrolytic salt and a solvent.

In some implementations, the electrolytic salt may be selected from oneor more of LiPF₆ (lithium hexafluorophosphate), LiBF₄ (lithiumtetrafluoroborate), LiClO₄ (lithium perchlorate), LiAsF₆ (lithiumhexafluoroborate), LiFSI (lithium bisfluorosulfonyl imide), LiTFSI(lithium bis-trifluoromethanesulfon imide), LiTFS (lithiumtrifluoromethanesulfonat), LiDFOB (lithium difluorooxalatoborate), LiBOB(lithium bisoxalatoborate), LiPO₂F₂ (lithium difluorophosphate), LiDFOP(lithium difluorophosphate), and LiTFOP (lithium tetrafluoro oxalatephosphate).

In some implementations, the solvent may be selected from one or more ofethylene carbonate (EC), propylene carbonate (PC), ethyl methylcarbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC),dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propylcarbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate(FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA),propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP),propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB),1,4-butyrolactone (GBL), sulfolane (SF), methyl sulfonyl methane (MSM),methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

In some implementations, the electrolyte further optionally includes anadditive. For example, the additive may include a negative-electrodefilm forming additive, or may include a positive-electrode film formingadditive, or may include an additive capable of improving someperformance of a battery, for example, an additive for improvingover-charge performance of the battery, an additive for improvinghigh-temperature performance of the battery, and an additive forimproving low-temperature performance of the battery.

[Separator]

In secondary batteries using the liquid electrolyte and some secondarybatteries using the solid electrolyte, separators are also included. Theseparator is provided between the positive-electrode plate and thenegative-electrode plate, and plays a role of isolation. There is noparticular limitation on the type of the separator in this application,and any known porous-structure separator with good chemical stabilityand mechanical stability can be selected. In some implementations, amaterial of the separator may be selected from one or more of glassfiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidenefluoride. The separator may be a single-layer thin film or a multi-layercomposite film. When the separator is a multi-layer composite film, eachlayer may be made of the same or different materials.

In some implementations, the positive-electrode plate, thenegative-electrode plate, and the separator may be made into anelectrode assembly through wounding or lamination.

In some implementations, the secondary battery may include an outerpackage. The outer package may be used to encapsulate the foregoingelectrode assembly and electrolyte.

In some implementations, the outer package of the secondary battery maybe a hard shell, for example, a hard plastic shell, an aluminum shell,or a steel shell. The outer package of the secondary battery mayalternatively be a soft pack, for example, a soft pouch. A material ofthe soft pack may be plastic, for example, one or more of polypropylene(PP), polybutylene terephthalate (PBT), polybutylene succinate (PBS),and the like.

This application does not impose any special limitations on a shape ofthe secondary battery, and the lithium-ion battery may be of acylindrical shape, a rectangular shape, or any other shapes. FIG. 5shows a secondary battery 5 with a rectangular structure in an example.

In some implementations, referring to FIG. 6 , the outer package mayinclude a housing 51 and a cover plate 53. The housing 51 may include abase plate and side plates connected to the base plate, and the baseplate and the side plates enclose an accommodating cavity. The housing51 has an opening communicating with the accommodating cavity, and thecover plate 53 covers the opening to close the accommodating cavity. Apositive-electrode plate, a negative-electrode plate, and a separatormay be made into an electrode assembly 52 through winding or lamination.The electrode assembly 52 is encapsulated into the accommodating cavity.The electrolyte is infiltrated into the electrode assembly 52. There maybe one or more electrode assemblies 52 in the secondary battery 5, andthe quantity may be adjusted as required.

In some implementations, the secondary batteries may be assembled into abattery module. The battery module may include a plurality of secondarybatteries, and a specific quantity may be adjusted based on applicationand capacity of the battery module.

FIG. 7 shows a battery module 4 in an example. Referring to FIG. 7 , inthe battery module 4, a plurality of secondary batteries 5 may besequentially arranged in a length direction of the battery module 4.Certainly, an arrangement may be made in any other manner. Further, theplurality of secondary batteries 5 may be fastened through fasteners.

Optionally, the battery module 4 may further include a housing with anaccommodating space, and the plurality of secondary batteries 5 areaccommodated in the accommodating space.

In some implementations, the battery module may be further assembledinto a battery pack, and a quantity of battery modules included in thebattery pack may be adjusted based on application and capacity of thebattery pack.

FIG. 8 and FIG. 9 show a battery pack 1 in an example. Referring to FIG.8 and FIG. 9 , the battery pack 1 may include a battery box and aplurality of battery modules 4 arranged in the battery box. The batterybox includes an upper box body 2 and a lower box body 3. The upper boxbody 2 is configured to cover the lower box body 3 to form a closedspace for accommodating the battery modules 4. The plurality of batterymodules 4 may be arranged in the battery box in any manner.

Apparatus

This application further provides an apparatus. The apparatus includesat least one of the secondary battery, the battery module, or thebattery pack according to this application. The secondary battery, thebattery module, or the battery pack may be used as a power source of theapparatus, or an energy storage unit of the apparatus. The apparatus maybe, but is not limited to, a mobile device (for example, a mobile phoneor a notebook computer), an electric vehicle (for example, a batteryelectric vehicle, a hybrid electric vehicle, a plug-in hybrid electricvehicle, an electric bicycle, an electric scooter, an electric golfvehicle, or an electric truck), an electric train, a ship, a satellite,an energy storage system, and the like. A secondary battery, a batterymodule, or a battery pack may be selected for the apparatus according torequirements for using the apparatus.

FIG. 10 shows an apparatus in an example. The apparatus is a batteryelectric vehicle, a hybrid electric vehicle, a plug-in hybrid electricvehicle, or the like. To meet requirements of the apparatus for highpower and a high energy density, a battery pack or a battery module maybe used.

In another example, the apparatus may be a mobile phone, a tabletcomputer, a notebook computer, or the like. The apparatus is usuallyrequired to be light and thin, and the secondary battery may be used asa power source.

EXAMPLES

Content disclosed in this application is described in more details inthe following examples. These examples are intended only forillustrative purposes because various modifications and changes madewithout departing from the scope of the content disclosed in thisapplication are apparent to those skilled in the art. Unless otherwisestated, all parts, percentages, and ratios reported in the followingexamples are based on weights, all reagents used in the examples arecommercially available or synthesized in a conventional manner, and canbe used directly without further processing, and all instruments used inthe examples are commercially available.

I. Preparation of the Battery

Example 1

Preparation of the Negative-Electrode Active Material

Petroleum green coke was used, with a volatile content C₁ being 7.87%.The petroleum green coke was pulverized to obtain a coke raw materialwith D_(v)50 of 11.8 μm and D_(v)99 of 20.1 μm.

Shaping and classification were performed on the coke raw material toobtain a precursor with D_(v)50 of 13.0 μm and D_(v)99 of 21.3 μm.

The precursor was granulated by using a binder pitch, and an amount ofthe binder C₂ was 5%. The resulting granulated product had D_(v)50 of13.7 μm and D_(v)99 of 21.9 μm.

Graphitization was performed on the granulated product at a temperatureof 3000° C., followed by sieving, to obtain artificial graphite. D_(v)99of the artificial graphite was 22.9 μm.

Then, the artificial graphite was coated with the organic carbon sourcepitch, graphitization treatment was performed, to obtain anegative-electrode active material, including an artificial graphitecore and an amorphous carbon coating layer covering a surface of theartificial graphite core. An amount of the organic carbon source C₃ was3%, and the negative-electrode active material satisfied D_(v)50 of 14.5μm, D_(v)99 of 22.3 μm, and a gram capacity of 355.2 mAh/g.

Preparation of the Negative-Electrode Plate

The negative-electrode active material prepared above, a binderstyrene-butadiene rubber (SBR), a thickener sodium carboxymethylcellulose (CMC-Na), and a conductive agent carbon black (Super P) werefully stirred and mixed in an appropriate amount of deionized water in aweight ratio of 96.2:1.8:1.2:0.8, to form a uniform negative-electrodeslurry; and the negative-electrode slurry was applied on a surface of acopper foil negative-electrode current collector, followed by drying,cold pressing, slitting, and cutting, to obtain a negative-electrodeplate. A compacted density of the negative-electrode plate was 1.65g/cm³, and an areal density was 123 g/m².

Preparation of the Positive-Electrode Plate

Lithium nickel cobalt manganese oxideLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂(NCM811), a conductive agent carbon black(Super P), and a binder PVDF were fully stirred and mixed in anappropriate amount of N-methylpyrrolidone (NMP) in a weight ratio97.5:1.5:1, to form a uniform positive-electrode slurry; and thepositive-electrode slurry was applied on a surface of an aluminum foilpositive-electrode current collector, followed by drying, cold pressing,slitting, and cutting, to obtain a positive-electrode plate. A compacteddensity of the positive-electrode plate was 3.5 g/cm³, and an arealdensity was 196 g/m².

Separator

A polyethylene (PE) film was used as a separator.

Preparation of the Electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethylcarbonate (DEC) were mixed at a volume ratio of 1:1:1, and then fullydried lithium salt LiPF₆ was uniformly dissolved in the foregoingsolution to obtain an electrolyte, where the concentration of LiPF₆ was1 μmol/L.

Preparation of the Secondary Battery

A positive-electrode plate, a separator, and a negative-electrode platewere stacked in order, and a reference electrode (where the referenceelectrode is used for subsequent performance testing of battery samples,and may be selected from a lithium sheet, lithium metal wire, or thelike; and the reference electrode needs to be separated by the separatorto avoid coming in contact with either side of the positive-electrodeplate and the negative-electrode plate) was added between the separatorand the negative-electrode plate. The stack was made into an electrodeassembly through winding, the electrode assembly was encapsulated intoan outer package, and the electrolyte was added, followed by theprocesses of encapsulation, standing, chemical formation, aging, and thelike, to obtain a secondary battery.

The preparation methods in Examples 2 to 20 are similar to those inComparative Example 1, with the only difference in preparationparameters of the negative-electrode active material. For details aboutdifferent preparation parameters and product parameters, refer to Table2 to Table 5.

II. Battery Performance Testing

(1) Fast Charging Performance Testing

The secondary batteries prepared in the Examples and ComparativeExamples were charged at 25° C. to 4.25V at a constant current of 1C(that is, a current value at which a theoretical capacity is completelydischarged in 1 h), charged to a current of 0.05C at a constant voltage,left standing for 5 min, and then discharged to 2.8V at a constantcurrent of 1C; and an actual capacity was recorded as Co.

Then, the batteries were charged to 4.25V or 0V negative cut-offpotential (whichever comes first) successively at a constant current of0.5C₀, 1C₀, 1.5C₀, 2C₀, 2.5C₀, 3C₀, 3.5C₀, 4C₀, and 4.5C₀, and needed tobe discharged to 2.8V at 1C₀ after each charge was completed.Corresponding negative potentials for charging to 10%, 20%, 30%, . . . ,and 80% SOC (state of charge) at different charging rates were recorded,and rate-negative potential curves in different SOC states were drawn.Charging rates corresponding to the negative potential of 0V indifferent SOC states were obtained through linear fitting, and weredenoted by C_(20% SOC), C_(30% SOC), C_(40% SOC), C_(50% SOC),C_(60% SOC), C_(70% SOC), and C_(80% SOC). According to a formula(60/C_(20% SOC)+60/C_(30% SOC)+60/C_(40% SOC)+60/C_(50% SOC)+60/C_(60% SOC)+60/C_(70% SOC)+60/C_(80% SOC))×10%,a charging time T (min) for charging the battery from 10% SOC to 80% SOCwas obtained through calculation. A shorter time indicates better fastcharging performance of the battery.

(2) Cycling Performance Testing

The secondary batteries prepared in the Examples and ComparativeExamples were charged at 25° C. to 4.25V at a constant current of 0.33C,charged to a current of 0.05C at a constant voltage, left standing for 5min, and then discharged to 2.8V at a constant current of 0.33C; and aninitial capacity was recorded as C₀. Then, the batteries were chargedaccording to strategies in Table 1, and discharged at 0.33C. Adischarging capacity C_(n) of each cycle was recorded, until a cyclingcapacity retention rate (C_(n)/C₀×100%) became 80%; and then the numberof cycles was recorded. A larger quantity of cycles indicates a longercycle life of the battery.

TABLE 1 State of charge Charging SOC of battery rate (C)    0-10% 0.3310%-20% 3.8 20%-30% 2.9 30%-40% 2.4 40%-50% 2.0 50%-60% 1.7 60%-70% 1.470%-80% 1.2  80%-100% 0.33

For details about test results of Examples 1 to 20 and ComparativeExamples 1 and 2, refer to Table 3 and Table 5.

TABLE 2 Preparation Parameters Artificial graphite obtained CoatingGranulated through C₃ × Coke raw material Precursor productgraphitization carbon D_(v)50 D_(v)99 C₁ D_(v)50 D_(v)99 D_(v)50 D_(v)99D_(v)50 D_(v)99 C₃ residue Sequence number (μm) (μm) (%) (μm) (μm) (μm)(μm) (μm) (μm) (%) rate (%) Example 1 11.8 20.1 7.87 13.0 21.3 13.7 21.912.2 22.9 3.00 2.10 Example 2 11.2 18.7 7.15 12.2 19.9 12.9 20.1 11.421.8 3.30 2.31 Example 3 9.7 17.6 7.56 11.3 18.9 11.8 19.0 10.2 20.83.00 2.10 Example 4 8.2 17.5 7.43 9.8 18.2 10.5 19.7 9.3 20.0 3.50 2.45Example 5 7.5 17.1 7.00 9.3 17.9 10.1 19.2 8.8 19.6 3.00 2.10 Example 67.3 16.4 7.88 8.9 17.0 9.7 18.3 8.5 18.5 3.80 2.66 Example 7 7.1 15.96.67 8.5 16.4 9.2 17.1 7.5 17.5 2.20 1.53 Comparative 12.3 30.0 6.7613.2 34.0 14.2 38.3 12.7 38.6 3.00 2.10 Example 1 Comparative 15.1 36.46.23 15.1 38.1 16.3 47.6 14.8 48.1 3.20 2.24 Example 2

TABLE 3 Test Results Negative-electrode Battery performance activematerial Fast Cycling Gram charging performance Sequence D_(v)99 D_(v)50Capacity performance (number of number (μm) (μm) (mAh/g) (min) cycles)Example 1 22.3 14.5 355.2 20.8 1950 Example 2 21.4 13.2 354.3 18.5 2320Example 3 20.3 12.4 353.6 17.4 2530 Example 4 18.9 11.4 351.3 20.2 2005Example 5 18.4 10.5 350.7 20.4 1930 Example 6 17.8 10.2 350.5 20.6 1830Example 7 16.3 8.3 348.7 18.8 2050 Comparative 36.0 12.0 353.2 23.7 907Example 1 Comparative 45.2 17.5 359.4 28.5 1270 Example 2

As can be seen from the results in Table 3, the negative-electrodeactive material of this application includes the core and the coatinglayer covering the surface of the core, the core includes artificialgraphite, the coating layer includes amorphous carbon, and thenegative-electrode active material satisfies D_(v)99≤24 μm and 8μm≤D_(v)50≤15 μm, thereby improving the fast-charging capability andcycling performance of the secondary battery using thenegative-electrode active material under the condition of higher energydensity.

In Comparative Example 1, both the fast-charging capability and thecycling performance of the secondary battery are relatively poor becausethe foregoing conditions are not satisfied.

TABLE 4 Preparation Parameters Artificial Granulation graphite CoatingD_(v)50 D_(v)99 obtained C₃ × of gran- of gran- through carbon Coke rawmaterial Precursor ulated ulated graphitization residue Sequence D_(v)50D_(v)99 C₁ D_(v)50 D_(v)99 C₂ A product product D_(v)50 D_(v)99 C₃ rateB number Type (μm) (μm) (%) (μm) (μm) U₁ (%) (%) (μm) (μm) (μm) (μm) U₂(%) (%) (%) Example Petroleum  9.6 16.1 7.12 10.8 17.2 0.32 5.13 37.8812.5 18.1 11.1 19.3 0.27 3.00 2.1  56.00 7 green coke Example Petroleum 9.2 17.2 7.56 10.3 18.3 0.38 5.25 33.05 11.9 19.2 10.5 20.3 0.33 3.002.1  47.15 8 green coke Example Petroleum  9.2 17.6 7.88 10.1 18.6 0.425.45 30.67 12.2 20.1 10.6 21.6 0.38 3.00 2.1  41.79 9 green coke ExamplePetroleum  9.4 19.2 7.14 10.5 20.3 0.45 5.16 26.98 12.0 21.3 10.3 22.40.39 3.00 2.1  38.82 10 green coke Example Petroleum  9.7 20.3 7.98 11.021.4 0.49 5.76 26.49 12.8 22.4 11.3 23.2 0.45 3.00 2.1  35.51 11 greencoke Example Petroleum  9.9 18.4 7.56 11.2 20.2 0.55 6.21 25.02 13.023.2 11.9 23.1 0.36 3.30 2.31 47.39 12 green coke Example Petroleum  9.218.1 8.23 10.2 19.3 0.48 5.72 29.02 12.4 22.1 11.0 20.3 0.35 2.80 1.9547.80 13 green coke Example Petroleum 10.5 17.5 7.45 11.6 18.5 0.43 4.3127.33 12.9 18.6 11.5 21.2 0.35 3.20 2.24 42.71 14 green coke ExamplePetroleum 10.8 19.2 8.54 11.8 20.1 0.35 4.67 37.54 13.4 20.7 11.9 22.30.33 2.90 2.03 48.61 15 green coke Example Petroleum 10.1 20.4 6.78 11.221.1 0.33 2.25 27.21 12.8 21.4 11.7 23.4 0.35 3.30 2.34 35.09 16 greencoke Example Petroleum  7.1 16.8 6.54  8.7 18.3 0.49 9.15 32.70 10.222.3  9.2 23.5 0.46 1.80 1.26 38.70 17 green coke Example Petroleum  9.317.3 7.34 10.5 18.5 0.38 5.34 31.58 11.9 19.2 10.6 20.6 0.33 3.00 2.1045.45 18 needle coke Example Petroleum  9.4 17.5 6.54 10.7 17.9 0.434.36 25.21 12.3 18.6 10.9 20.1 0.35 3.20 2.27 40.11 19 needle coke +petroleum green coke Example Calcined  9.6 17.2 8.54 10.9 18.2 0.34 2.6142.90 12.7 18.9 11.3 19.7 0.31 2.20 1.53 54.20 20 petroleum coke

In Table 4, A=(C₁+C₂)/U₁×1000%; and B=(C₁+C₂+C₃)/U₂×100%.

TABLE 5 Test Results Negative-electrode active material Particle sizeQuantity Battery performance specific proportion Fast Cycling Particlesurface of the Gram charging performance Sequence D_(v)99 D_(v)50 sizearea secondary capacity performance (number of number (μm) (μm)uniformity (m²/g) particles (mAh/g) (min) cycles) Example 7 19.2 12.30.25 0.62 80% 353.6 21.0 1980 Example 8 20.1 12.6 0.32 0.53 80% 353.417.3 2530 Example 9 21.3 12.1 0.35 0.5  80% 353.8 18.5 2320 Example 1022.3 11.3 0.38 0.47 80% 352.6 20.2 2005 Example 11 23.1 11.2 0.42 0.4580% 352.4 20.6 1830 Example 12 23.6 14.1 0.34 0.68 90% 353.2 21.5 2145Example 13 22.2 13.5 0.34 0.62 85% 353.7 20.7 2217 Example 14 21.3 12.20.34 0.51 75% 353.3 18.9 2401 Example 15 22.2 11.6 0.34 0.46 73% 352.318.7 2524 Example 16 23.1 12.3 0.34 0.42 72% 352.4 19.5 2430 Example 1723.2 13.2 0.4  0.71 100%  352.8 18.5 1901 Example 18 20.3 12.6 0.32 0.5380% 353.3 17.4 2530 Example 19 20.4 12.3 0.34 0.51 75% 352.1 18.9 2401Example 20 20.5 11.3 0.3  0.48 60% 351.7 20.6 2178

It can be seen from the results of Examples 7 to 11 that thenegative-electrode active material with the particle size uniformityfurther being within an appropriate range can further improve thefast-charging capability and cycling performance of the battery.

It can be seen from the results of Examples 12 to 20 that thenegative-electrode active material with the particle size specificsurface area or the quantity proportion of the secondary particlesfurther being within an appropriate range can further improve thefast-charging capability and cycling performance of the battery.

The foregoing descriptions are merely specific embodiments of thisapplication, but are not intended to limit the protection scope of thisapplication. Any equivalent modifications or replacements readilyfigured out by a person skilled in the art within the technical scopedisclosed in this application shall fall within the protection scope ofthis application. Therefore, the protection scope of this applicationshall be subject to the protection scope of the claims.

What is claimed is:
 1. A negative-electrode active material, comprisinga core and a coating layer covering at least part of a surface of thecore, wherein the core comprises artificial graphite, and the coatinglayer comprises amorphous carbon; wherein a volume-based particle sizedistribution of the negative-electrode active material satisfiesD_(v)99≤24 μm, and a volume-based median particle size D_(v)50 of thenegative-electrode active material satisfies 8 μm≤D_(v)50≤15 μm; whereinD_(v)99 is a particle size corresponding to a cumulative volumedistribution percentage of the negative-electrode active materialreaching 99%, and D_(v)50 is a particle size corresponding to acumulative volume distribution percentage of the negative-electrodeactive material reaching 50%.
 2. The negative-electrode active materialaccording to claim 1, wherein the volume-based particle sizedistribution D_(v)99 satisfies 17 μm≤D_(v)99≤24 μm.
 3. Thenegative-electrode active material according to claim 1, wherein thevolume-based median particle size D_(v)50 satisfies 9 μm≤D_(v)50≤13 μm.4. The negative-electrode active material according to claim 1, whereinparticle size uniformity of the negative-electrode active material ismeasured by a unit-less quantity (U); and wherein particle sizeuniformity of the negative-electrode active material is 0.25-0.45. 5.The negative-electrode active material according to claim 1, wherein aparticle size specific surface area of the negative-electrode activematerial is 0.4 μm²/g-0.75 μm²/g.
 6. The negative-electrode activematerial according to claim 1, wherein the negative-electrode activematerial further comprises secondary particles, and a quantityproportion of the secondary particles in the negative-electrode activematerial is ≥50%.
 7. The negative-electrode active material according toclaim 1, wherein the negative-electrode active material satisfies one ormore of the following conditions: 0.6≤(D_(v)90−D_(v)10)/D_(v)50≤1.8,wherein D_(v)90 is a particle size corresponding to a cumulative volumedistribution percentage of the negative-electrode active materialreaching 90%, and D_(v)10 is a particle size corresponding to acumulative volume distribution percentage of the negative-electrodeactive material reaching 10%; the volume-based particle sizedistribution D_(v)90 of the negative-electrode active material is 13μm-18 μm; and the volume-based particle size distribution D_(v)10 of thenegative-electrode active material is 5 μm-10 μm.
 8. Thenegative-electrode active material according to claim 1, wherein thenegative-electrode active material satisfies one or more of thefollowing conditions: a graphitization degree of the negative-electrodeactive material is 91.0%-96.0%; a gram capacity of thenegative-electrode active material is 345 mAh/g-360 mAh/g; a tap densityof the negative-electrode active material is 0.9 g/cm³-1.3 g/cm³; and apowder compacted density of the negative-electrode active material undera pressure of 2 kN is 1.55 g/cm³-1.67 g/cm³.
 9. A secondary battery,comprising a negative-electrode plate, wherein the negative-electrodeplate comprises the negative-electrode active material according toclaim 1.